How do you write tests for the argparse portion of a python module?

Question

I have a Python module that uses the argparse library. How do I write tests for that section of the code base?

Answer 1

You should refactor your code and move the parsing to a function:

def parse_args(args):
    parser = argparse.ArgumentParser(...)
    parser.add_argument...
    # ...Create your parser as you like...
    return parser.parse_args(args)

Then in your main function you should just call it with:

parser = parse_args(sys.argv[1:])

(where the first element of sys.argv that represents the script name is removed to not send it as an additional switch during CLI operation.)

In your tests, you can then call the parser function with whatever list of arguments you want to test it with:

def test_parser(self):
    parser = parse_args(['-l', '-m'])
    self.assertTrue(parser.long)
    # ...and so on.

This way you'll never have to execute the code of your application just to test the parser.

If you need to change and/or add options to your parser later in your application, then create a factory method:

def create_parser():
    parser = argparse.ArgumentParser(...)
    parser.add_argument...
    # ...Create your parser as you like...
    return parser

You can later manipulate it if you want, and a test could look like:

class ParserTest(unittest.TestCase):
    def setUp(self):
        self.parser = create_parser()

    def test_something(self):
        parsed = self.parser.parse_args(['--something', 'test'])
        self.assertEqual(parsed.something, 'test')